Solid core golf balls are well known in the art. Typically, the core is made from polybutadiene rubber material, which provides the primary source of resiliency for the golf ball. U.S. Pat. Nos. 3,241,834 and 3,313,545 disclose the early work in polybutadiene chemistry. It is also known in the art that increasing the cross-link density of polybutadiene can increase the resiliency of the core. The core is typically protected by a cover from repeated impacts from golf clubs. The golf ball may comprise additional layers, which can be an outer core or an inner cover layer. One or more of these additional layers may be a wound layer of stretched elastic windings to increase the ball's resiliency.
A known drawback of polybutadiene cores cross-linked with peroxide and/or zinc diacrylate is that this material is adversely affected by moisture. Water moisture vapor reduces the resiliency of the cores and degrades its properties. A polybutadiene core will absorb water and loose its resilience. Thus, these cores must be covered quickly to maintain optimum ball properties. The cover is typically made from ionomer resins, balata, and urethane, among other materials. The ionomer covers, particularly the harder ionomers, offer some protection against the penetration of water vapor. However, it is more difficult to control or impart spin to balls with hard covers. Conventional urethane covers, on the other hand, while providing better ball control, offer less resistance to water vapor than ionomer covers.
Prolonged exposure to high humidity and elevated temperature may be sufficient to allow water vapor to invade the cores of some commercially available golf balls. For example, at 110° F. and 90% humidity for a sixty day period, significant amounts of moisture enter the cores and reduce the initial velocity of the balls by 1.8 ft/s to 4.0 ft/s or greater. The change in compression may vary from 5 PGA to about 10 PGA or greater. The absorbed water vapor also reduces the coefficient of restitution (CoR) of the ball.
Several prior patents have addressed the water vapor absorption problem. U.S. Pat. No. 5,820,488 discloses a golf ball with a solid inner core, an outer core and a water vapor barrier layer disposed therebetween. The water vapor barrier layer preferably has a water vapor transmission rate lower than that of the cover layer. The water vapor barrier layer can be a polyvinylidene chloride (PVDC) layer. It can also be formed by an in situ reaction between a barrier-forming material and the outer surface of the core. Alternatively, the water vapor barrier layer can be a vermiculite layer. U.S. Pat. Nos. 5,885,172 and 6,132,324 disclose, among other things, a golf ball with a polybutadiene or wound core with an ionomer resin inner cover and a relatively soft outer cover. The hard ionomer inner cover offers some resistance to water vapor penetration and the soft outer cover provides the desirable ball control. Additionally, U.S. Pat. No. 5,875,891 discloses an impermeable packaging for golf balls. The impermeable packaging acts as a moisture barrier to limit moisture absorption by golf balls during storage, but not during use.
The moisture vapor barrier layer disclosed in the prior patents can be rigid and makes the ball stiffer. Furthermore, producing a rigid layer may cause significant production obstacles. On the other hand, less rigid polymers, such as butyl rubber and other rubbers, are known to have low permeability to air, gases and moisture. Butyl rubber is widely used as sealant for rooftops, as inner liner in tubeless tires, and as lining for chemical tanks, among other uses. In the golf ball art, butyl rubber's usage has been limited to practice balls or driving range balls due to its slow initial velocity and low CoR, as discussed in U.S. Pat. Nos. 5,209,485 and 4,995,613. Butyl rubber is also used as the outermost cover layer or a part of the cover due to its durability, as disclosed in U.S. Pat. Nos. 5,873,796 and 5,882,567, among others. However, the moisture vapor barrier advantage of butyl rubber has not heretofore been utilized in the golf ball art to make a better performing golf ball.
Also, high-temperature curing of certain polymeric materials to form the water vapor barrier layer or other outer layers on the golf ball can be difficult to accomplish, since such curing or crosslinking heats the entire golf ball subassembly. This heating method may degrade the untargeted components within the subassembly. Additionally, this curing method limits suitable outer layer materials to materials having a cured temperature that is lower than the softening temperature or lower melting temperature of the inner layers or core.
Hence, there remains a need for a golf ball with an improved water vapor barrier layer and having varying hardness within the core of the golf ball.